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Utatsusaurus
Utatsusaurus is the earliest-known form of an ichthyopterygian (the group of marine reptiles that includes the ichthyosaurs), which lived in the early Triassic period (c. 245-250 millions years ago). Unlike the more advanced ichthyosaurs, Utatsusaurus has no dorsal fin and has a broad skull that tapers slowly toward the snout. For the size of the skull, the teeth are rather small, and arranged in a primitive groove. Utatsusaurus had small fins, with four toes instead of the usual five found in primitive ichthyosaurs. The tail had a long low fin, suggesting that the animal swam by undulation, rather than using its paddles and tail. Feeding on a diet of fish, Utatsusaurus reached a length of three meters (nine feet). Utatsusaurus has been found in Japan and Canada. Ryosuke Motani from the University of California, Berkeley and Nachio Minoura from Hokkaido University re-examined the fossils of Utatsusaurus in 1998 using computer imagery to reverse the distortion of the original skeleton. They found that Utatsusaurus was closely related to the lizard-like diapsid reptiles such as Petrolacosaurus, making ichthyopterygians a distant relative to lizards, snakes and crocodiles. Description Unlike the more advanced ichthyosaurs, Utatsusaurus has no dorsal fin and has a broad skull. The snout gently tapers, compared to the more rounded one of more derived ichthyopterygians.6The postorbital underlaps the elongate posterior process of the postfrontal. This is an evident plesiomorphic condition for ichthyopterygians.6 For the size of the skull, the teeth are rather small, and arranged in a primitive groove. They have longitudinal grooves and were first thought to be longer and more acute than Grippia, which is a closely related ichthyosaur.2 But, after that, it was reported that they were rather bluntly pointed and robust by reexamining the holotype.7 Utatsusaurus had small fins, with five digits.4 In addition, those digits have up to five extra finger bones, which is referred to as hyperphalangy.4 5The tail had a long low fin, suggesting that the animal swam by undulation, rather than using its paddles and tail. Utatsusaurus has transitional features between ancestral terrestrial amniotes and the more derived ichthyosaurs. First, the attachment of the pelvic girdle to the vertebral column was probably not robust enough to support the body on land unlike terrestrial amniotes. The pelvic girdle is attached to the vertebral column by the sacral ribs probably articulating with the ilium, but the ribs are not fused to the sacral vertebrae. Second, the humerus and femur of Utatsusaurus has the equal length. While all other ichthyosaurs have the longer humerus, terrestrial amniotes have the longer femur. Furthermore, the hindlimb of Utatsusaurus seems to be larger than the forelimb.1 They also used phylogenetic analyses and concluded that ichthyosaurs were a member of the Diapsida and the sister group of the Sauria. Paleobiology Utatsusaurus fed on a diet of fish. It has approximately 40 presacral vertebrae which are cylindrical, suggesting that it probably swam with an eel-like motion. Classification References * . Category:Triassic ichthyosaurs Category:Prehistoric reptiles of Asia Category:Prehistoric reptiles of North America